Laser printers are used in a wide variety of applications including office printing, medical printing, and bar code printing. These laser printers are available in a number of configurations depending on the use of the printer. For example, a flying spot printer typically uses a single, low power beam, to print to light sensitive media. It is often desirable, however, to print to a broader variety of media and with a higher pixel density.
Systems used in the graphic arts industry, for example, are configured as multispot printers to attain high productivity. Since most of the graphic arts media is relatively insensitive to light exposure, each of these incident beams must provide a high light level to a small spot at the media plane. These printers are typically configured like a lathe, where the page scan is obtained by rotating a drum which holds the media, and line scan by translating the multiple laser beams in a direction parallel to the axis of rotation of the drum.
There are several approaches to produce multiple writing spots in graphic arts printer designs. In one approach, each of the laser sources is separately coupled to optical fibers, which are then mounted to form a linear array of sources. Each of these channels can then be independently modulated. Such systems are described in U.S. Pat. Nos. 4,900,130 and 5,351,617. Another approach is to utilize a monolithic array of laser sources and then image the elements of the laser array directly onto the light sensitive media to produce multiple spots. Power to each element of the laser array is individually modulated to obtain modulator site densities. Such a system is described in U.S. Pat. No. 4,804,975 and is potentially of lower cost and higher efficiency than systems which couple lasers to optical fibers.
A monolithic printing system with a diode array source can be improved by splitting each lasing element into an array of subarray sources, such as described in U.S. Pat. No. 5,619,245. Thus, each modulator site is comprised of the combined light of all the lasing elements of a given subarray, and each of the subarrays are modulated to provide the image data input.
Another approach to improving a system with a monolithic diode array source is to combine the light from each lasing element to flood illuminate a linear spatial light modulator array. The modulator breaks up the light into image elements, and each modulator site is subsequently imaged onto the media plane to form the desired array of printing spots. Printing systems employing this approach are described in U.S. Pat. No. 4,786,918; U.S. Pat. No. 5,517,359; and U.S. Pat. No. 5,521,748. These systems improve on prior art designs by providing indirect light modulation, so that the laser diode array operates at full power, and serves only as a light source. Also, since light from the emitters overlap at the modulator, the resulting redundancy desensitizes the system to the failure of any the lasing elements within the array.
The performance of a system in which a linear spatial light modulator array is flood illuminated is highly dependent on both the design of the illumination system and the design and operation of the modulator array. Optimally, the illumination system should provide highly uniform illumination with minimal loss of brightness. In U.S. Pat. No. 4,786,918, the Gaussian beams from many single mode lasers are combined in the far field to create a broad and generally slowly varying illumination profile, but one which still falls off in a generally Gaussian manner. U.S. Pat. No. 5,517,359 provides for a printing system with a laser diode array consisting of multimode emitters, each of which typically has a non-uniform near field profile. A mirror system included in the illumination optics partially improves the light uniformity by substantially removing the macro-nonuniformities in the light profile. Another method, as described in U.S. patent application Ser. No. 08/757,889, filed Nov. 27, 1996, and assigned to the same assignee as the present invention, also describes a printing system with a laser diode array consisting of multimode emitters, but with an illumination system utilizing a fly's eye integrator. With the fly's eye integrator, both the micro and macro light non-uniformity can be substantially improved.
Since the illumination optics provide a uniform illumination of a linear spatial light modulator array, the overall system performance is highly dependent on the design and operation of the spatial light modulator array. Whether these devices are transmissive or reflective, light absorbing, light blocking, polarization altering, deflecting, or diffracting, a significant feature of their design is the presence of a high optical fill factor. That is, that the ratio of the width of the modulator site to the modulator site pitch approaches 100%. This is particularly important for graphic arts printers, where maximal efficiency with modest contrast (typically 20:1) is optimal. Light that is lost between modulator sites will reduce the system efficiency and writing contrast, and further, may damage the device.
There are several examples of electro-optical spatial light modulator arrays which have potential value in the design of a laser thermal printer. U.S. Pat. No. 4,887,104 describes an electro-optical linear modulator array using PLZT, wherein the linear array is broken into two rows of parallel, but offset sites which modulate the polarization state of the incident light. When a time delayed printing scheme is used, the two rows effectively reconstruct lines of image data. While the printed modulator sites are closely packed, the optical fill factor relative to the illumination system is low, and the system is inefficient. U.S. Pat. No. 5,402,154 describes a linear modulator array which provides a 100% fill factor by placing one row of 50% fill modulator sites on one side of an electro-optic sheet material with a second row of 50% fill modulator sites on the other side, but offset from those one the first side. However, the two sided manufacturing and assembly process for this device is quite complex, and furthermore PLZT is a notoriously difficult material to work with.
Another well known architecture for a electro-optical spatial light modulator array is the Total Internal Reflectance (TIR) modulator, described in U.S. Pat. No. 4,281,904 and U.S. Pat. No. 4,376,568. These devices are transmissive schlieren modulators, produced from lithium niobate (LiNbO.sub.3) or lithium tantalate (LiTaO.sub.3), which provide a high optical fill factor. The electro-optical materials used in these devices are robust, and because they are schlieren devices which redirect rather than absorb the modulated light, these modulators work well in laser thermal printers. However, the structure imposes severe constraints on the design of the input optical system, because the incident light must be directed at grazing incidence. Furthermore, the proximity of the exiting modulated and unmodulated light is not optimal for an imaging optical system when a high Lagrange light source is used. Such modulators are not easily adapted to two-dimensional arrangements of modulator sites or to multiple line illumination. Multiple line modulators, such as the modulator described in U.S. Pat. No. 4,560,994, require extremely complicated coupling optics, or a severe reduction in optical efficiency. U.S. patent application Ser. No. 08/763,174, filed Dec. 10, 1996, entitled "Addressable Electro-Optic Modulator with Periodically Poled Domain Regions" deals with TIR modulators.
Modulators that employ a waveguide geometry, see U.S. Pat. No. 5,418,871, and can tolerate high optical power density have been proposed for use in printing applications. However, the optical powers used in "Dye Submission Printing Technologies" are too large for most single mode or even multimode waveguide electro-optical crystals. Specifically, use of waveguiding in conjunction with an electro-optic material offers advantages for contrast and handling optical power. However, use of waveguide geometry requires specific input illumination angles. Even in the best case, waveguides experience considerable coupling losses. Furthermore, a waveguide is restricted to a one-dimensional array with relatively small cross-sectional geometry.
There are electromechanical modulator technologies which have potential utility for a laser thermal printer. For example, the DMD (digital mirror device), described in U.S. Pat. No. 5,061,049 can be constructed with a high fill factor by methods which lend themselves to mass production, however, this device is susceptible to damage in the case of high power illumination. The grating light valve, described in U.S. Pat. No. 5,311,360 and U.S. Pat. No. 5,459,610, is also a electromechanical diffractive schlieren modulator with a high fill factor and has demonstrated both fast rise and fall times, and a high fill factor. However, both micro-mechanical devices are limited in their ability to handle high optical power. Additionally, while establishing reasonably fast modulation times, neither is able to match the high speed modulation ability of electro-optic crystals.
In high power application, such as laser thermal printing, the schlieren type modulators, such as the grating light valves and the TIR modulators, are advantageous as they do not absorb the incident light, but rather deflect the modulated light out of the optical system. However, these modulators have not been optimally configured for use in a laser printer where the linear modulator array is flood illuminated by the light from a laser diode array.